DEPOLARIZED SOLID-STATE DIODE-PUMPED LASER

Abstract: The disclosed invention, a laser and method for producing depolarized laser light, pertains to the field of laser radiation production. One embodiment of the laser comprises a solid state non-birefringent lasant material (18), a laser diode source (S) and birefringent optical retarding means (24) for transmitting light from the laser diode source to the lasant material, for reflecting laser light produced by the lasant material towards the output coupler (20) to form an optical cavity for the lasant material, and for forming in the cavity single line multimode laser light having at least two longitudinal standing laser modes characterized by two closely spaced apart orthogonal linear polarizations of approximately the same power level.

Description: DEPOLARIZED SOLID-STATE DIODE-PIJMPED LASER Technical Field This invention relates to the general subject of lasers and, in particular to diode-pumped, solid-state lasers and methods for producing a depolarized output. Background of the Invention Fiberoptic sensors using Mach-Zehnder modulators often require a stable, polarized optical carrier at the output end of a long length of variable birefringence optical fiber. This occurs in coherent communications and in fiber sensors to avoid input lead polarization noise and polarization fading. In general, three approaches have been employed to solve this problem: using polarization preserving fiber; active polarization control with a birefringence device which produces the desired state at the output in conjunction with a feedback loop; and using depolarized light which will always remain depolarized independent of fiber birefringence. (See A.D. Kersey, et al., Electron Lett.. 23; p. 634 and p. 924; 1987). The second method is illustrated by the system developed by Marshall, at EG&G Energy Measurements. That system uses discrete retardation plates located in front of the laser. A computer monitors the feedback signal and uses stepper motor rotation stages to adjust the launch polarization state. The third approach has the advantage of avoiding expensive polarization preserving fiber and does not require complex electronic feedback control loops. One convenient way of achieving this is by combining a depolarized optical source with a polarizer at the modulator input. This approach has been demonstrated using two polarization- coupled, frequency-offset, single-frequency lasers as a source. (See W.R. Burns, R.P. Moeller, CH. Buhner and A.S. Greenblatt, "Depolarized source for fiber-optic applications"; Opt. Lett.. 16(6), 381 (1991) and B. Marshall "Polarization Control Systems" Proceedings PSAA-91. pp. 83 (1991)). In particular, the system of Burns et al. used ring cavity, diode-pumped Nd:YAG lasers which have precise narrow linewidths that can be thermally tuned. These lasers are normally linearly polarized. The depolarized source was simply created by combining the beams from two lasers whose linear polarization states are at 90° to each other, and adjusting the thermal controllers so that the frequency difference between the two sources is at some large frequency, compared to the detection bandwidth of the system of interest. All of these methods are relatively complicated, and in many applications a simpler, single-laser, depolarized source would be desirable. A laser with a polarization ratio near unity and a mode spacing large enough to keep intermodulation products out of the signal band is preferred. Multi-longitudinal mode operation would be acceptable, provided that the overall linewidth is small enough to avoid polarization dispersion effects in the fiber. Although a linear cavity NdrYAG laser with no intracavity polarizers passively satisfies the polarization requirement, such a randomly polarized laser is not directly useful because frequency splittings of a few MHz between nominally degenerate modes are often introduced by stresses in the laser rod. Therefore, there is still room for improvement. Summary of the Invention A general object of the invention is to provide a diode-pumped, solid-state laser having a depolarized output. Yet another object of the invention is to provide a polarized optical carrier for a communications system utilizing an external optical modulator and a non-polarization maintaining optical fiber. Still another object of the invention is to provide an apparatus and method for producing laser light characterized by at least two longitudinal standing modes having two orthogonal linear polarizations of about the same power level. One specific object of the invention is to provide a Nd:YAG laser having a quarterwave plate to produce two polarization eigenstates of substantially equal intensity and a single line output. In accordance with one embodiment of the present invention, a laser apparatus is provided comprising: a solid-state minimally birefringent lasant material having an input face and an opposite output face; an output coupler disposed towards said output face; pumping means for pumping light into said input face of said lasant material to produce a population inversion therein; optical retarding means, having one face abutting said input face of said lasant material and having an opposite face coated for transmitting light from said pumping means to said lasant material and for reflecting laser light produced by said material towards said output coupler to form an optical cavity for said lasant material, for forming in said cavity single line multimode laser light having at least two longitudinal standing laser modes characterized by two closely spaced apart orthogonal linear polarizations of approximately the same power level; and means for avoiding high order transverse laser modes in said cavity. In addition a method is disclosed for providing a polarized optical carrier for use in a communications system of the type using an external modulator which operates in response to an optical carrier supplied through an optical fiber, comprising the steps of: forming an optical cavity for a lasant material having an input face and an opposite output face by locating an output coupler disposed towards said output face and by locating a quarterwave plate having one face abutting said input face and having an opposite face that is coated for transmitting optical pumping light to said lasant material and for reflecting laser light produced by said lasant material towards said output coupler, providing a plurality of semi-conductors that, in response to the flow of electrical current therethrough, produces optical pumping light; pumping said lasant material using said semi-conductors to produce a population inversion in said lasant material; substantially reducing high order transverse modes in said cavity; and adjusting said optical cavity to have an optical length that is proportional to one-half the longitudinal mode spacing of said laser light and to produce standing wave, substantially single frequency laser light that is characterized by two closely spaced apart orthogonal linear polarizations of about the same power level out of said output coupler for at least some power levels of said semi-conductors. Other advantages and features of the present invention will become readily apparent from the following detailed description of the invention, the embodiments described therein, from the claims, and from the accompanying drawings. Brief Description of the Drawings FIG. 1 is a schematic diagram of the laser that is the subject of the present invention; and FIG. 2 is depicts the performance of the laser illustrated in FIG. 1. Detailed Description While this invention is susceptible to embodiment in many different forms, there is shown in the drawings, and will herein be described in detail, one specific embodiment of the invention. It should be understood, however, that the present disclosure is to be considered an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated. It is to be understood that the terms "light", "light pulse" and "laser energy" are used in this application not as limited to the visible spectrum of electromagnetic radiation, but as in the broad sense of "radiant energy". Turning to FIG. 1, there is illustrated a diode-pumped solid-state laser 10, a non-polarization maintaining optical fiber 12, a polarizer 14 and an optical modulator 16. The laser 10 comprises a solid-state non- birefringent lasant material 18, an output coupler 20, an aperture plate 22, a quarterwave plate 24, and an input mirror 26. The lasant material 18 is optically pumped by a source S. Suitable optical pumping means or sources S include, but are not limited to, laser diodes, light-emitting diodes (including superluminescent diodes and superluminescent diode arrays) and laser diode arrays, together with any ancillary packaging or structures. For the purposes hereof, the term "optical pumping means" includes any heat sink, thermoelectric cooler or packaging associated with said laser diodes, light-emitting diodes and laser diode arrays. For example, such devices are commonly attached to a heat resistant and conductive heat sink and are packaged in a metal housing. For efficient operation, the pumping means S is desirably matched with a suitable absorption band of the lasant material 18. Although the invention is not to be so limited, a highly suitable optical pumping source consists of a gallium aluminum arsenide laser diode, which emits light having a wavelength of about 810 nm, that is attached to heat sink. Heat sink can be passive in character. However, heat sink can also compromise a thermoelectric cooler or other temperature regulation means to help maintain laser diode at a constant temperature and thereby ensure optimal operation of laser diode at a constant wavelength. It will be appreciated, of course, that during operation the optical pumping means will be attached to a suitable power supply. Electrical leads from laser diode, which are directed to a suitable power supply, are not illustrated in the drawings. Conventional light-emitting diodes and laser diodes are available which, as a function of composition, produce output radiation having a wavelength over the range from about 630 nm to about 1600 nm. Any device producing optical pumping radiation of a wavelength effective to pump a lasant material 18 can be used in the practice of this invention. For example, the wavelength of the output radiation from a GalnP based device can be varied from about 630 to about 700 nm by variation of the device composition. Similarly, the wavelength of the output radiation from a GaAlAs based device can be varied from about 750 to about 900 nm by variation of the device composition. InGaAsP based devices can be used to provide radiation in the wavelength range from about 1000 to about 1600 nm. If desired, the output facet of semiconductor light source S can be placed in butt-coupled relationship to input surface of the lasant material without the use of intermediate optics. As used herein, "butt-coupled" is defined to mean a coupling which is sufficiently close such that a divergent beam of optical pumping radiation emanating from semiconductor light source or laser diode S will optically pump a mode volume within the lasant material 18 with a sufficiently small transverse cross-sectional area so as to support essentially only single transverse mode laser operation (i.e., TEMQQ mode operation) in the lasant material. Suitable lasant materials 18 include, but are not limited to, solids selected from the group consisting of glassy and crystalline host materials which are doped with an active material and substances wherein the active material is a stoichiometric component of the lasant material. By way of specific example, neodymium-doped YAG or Nd:YAG is a highly suitable lasant material for use in combination with an optical pumping means S which produces light having a wavelength of about 808 nm. When pumped with light of this wavelength, neodymium-doped YAG can emit light having a wavelength of either about 1064 nm or about 1319 nm. Nd:YAG is a non-birefringent lasant material. Preferably the lasant material is "non-birefringent". By non-birefringent is meant that the gain medium has minimal birefringence in the direction of laser propagation. Thus, a birefringent material like Nd:YLF can be used provided that the laser propagates along an optic axis of the crystal where it is "effectively" nonbirefringent. The lasant material 18 is located in an optical cavity which is formed by the output coupler 20 and the input mirror 26. The lasant method has two opposite ends 18a and 18b. One end 18a faces the output coupler 20 and the opposite end 18b faces the input mirror 26. When pumped by the source S, a population inversion is produced and the lasant material produces laser light. The quarterwave plate 24 functions as a birefringent optical retarding means. It has one face 24a which abuts the input face 18b of the lasant material 18, and it has an opposite or outside face 24b which receives pumping light from the source S. In one embodiment, the outside face 24b is coated to function as the input mirror 26 of the laser cavity. In other words, the outside face 24b is coated to transmit pumping light from the source S and to reflect light, that is produced within the cavity, toward the output coupler 20. The quarterwave plate 24 also causes the laser to produce single line multimode laser light that is characterized by two closely spaced apart orthogonal linear polarization of approximately the same power level. The quarterwave plate 24 defines the polarization axes and introduces a half-wavelength path difference between the polarizations. This shifts the orthogonal-mode beat frequency to half the cavity mode spacing, giving splittings of several GHz. In principle, polarized scatter or absorption in the quarter-wave plate 24 could degrade the polarization ratio; this can be offset by polarization-sensitive spatial hole burning effects that bias the system towards depolarized operation. The aperture plate 22 is located between the lasant material 18 and the output coupler 20. Its purpose is to avoid the production of high order transverse modes in the cavity. If the source S comprises a plurality of laser diodes that are distributed along a reference axis (i.e., a diode array), the quarterwave plate 24 is located in the laser cavity to have its fast optical axis F aligned to the reference axis of the diode array. In one particular embodiment, the lasant material was Nd:YAG and the source S was a semi-conductor or diode array that produced pumping light at about 1318 nm. Its output was 200mW. Near threshold the laser operated in two closely-spaced, orthogonally-polarized modes (See Fig. 2). At higher powers, operation in several longitudinal modes was observed. The polarization ratio remained within a few percent of unity at all times. The modal beat frequency was above 2 GHz. The output of the laser 10 is coupled to the optical modulator 16 by means of a non-polarization maintaining single mode optical fiber 12 and a polarizer or polarization insensitive isolator 14. The polarizer produced plane polarized light (at about 150mW) for the carrier input to the modulator 16 (e.g., a Mach-Zehnder). From the foregoing description, it will be observed that numerous variations, alternatives and modifications will be apparent to those skilled in the art. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the invention. Various changes may be made, materials substituted and features of the invention may be utilized. For example, the source S can be focused by means of optics to pump the lasant material 18. It is also possible to introduce additional elements, such as etalons or birefringent filters, into the laser to restrict operation to only two modes. This amounts to extending the two-mode regime of operation, that has been observed at lower power, to higher power ranges. The laser with the etalon still uses a quarter-wave plate to control the cavity mode spacing and uses spatial hole burning to force two-mode operation. It is thus a simple modification of the apparatus just described. Thus, it will be appreciated that various modifications, alternatives, variations, etc., may be made without departing from the spirit and scope of the invention as defined in the appended claims. It is, of course, intended to cover by the appended claims all such modifications involved within the scope of the claims.

Claims: CLAIMS That which is claimed is: 1. Laser apparatus, comprising: a) a solid-state lasant material having an input face, an opposite output face, and a direction of propagation between said input face and said output face, said lasant material having minimal birefringence along said direction of propagation; b) an output coupler disposed towards said output face; c) pumping means for pumping light into said input face of said lasant material to produce a population inversion therein; d) optical retarding means, having one face abutting said input face of said lasant material and having an opposite face coated for transmitting light from said pumping means to said lasant material and for reflecting laser light produced by said material towards said output coupler to form an optical cavity for said lasant material, for forming in said cavity single line multimode laser light having at least two longitudinal standing laser modes characterized by two closely spaced apart orthogonal linear polarizations of approximately the same power level; and e) means for avoiding high order transverse laser modes in said cavity. 2. The apparatus of claim 1, further including: f) a non-polarization maintaining optical fiber having an input end connected to receive laser lightwave from said output coupler and having an opposite output end. 3. The apparatus of claim 2, further including: g) a polarizer for receiving light from said output end of said optical fiber and for producing a plane polarized light output. 4. The apparatus of claim 3, further including: h) an optical modulator for receiving light passing through said polarizer from said output end of said optical fiber. 5. The apparatus of claim 1, wherein said lasant material is non- birefringent. 6. The apparatus of claim 1, wherein said pumping means comprises an array of laser diodes that are distributed along a reference axis; and wherein said optical retarding means comprises a quarterwave plate having a fast optical axis aligned to said reference axis of said laser diode array and having a slow optical axis at right angles to said fast optical axis. 7. The apparatus of claim 1, wherein said cavity has an optical length that is proportional to one-half the mode spacing of said laser light. 8. The apparatus of claim 1, wherein said optical retarding means comprises a quarterwave plate whose fast and slow optical axes are in a plane parallel to said input face of said lasant material. 9. The apparatus of claim 8, wherein said optical retarding means includes at least one of an etalon and birefringent filter means. 10. The apparatus of claim 1, wherein said means for avoiding high order transverse laser modes in said cavity comprises an aperture plate located between said output coupler and said input face of said lasant material. 11. A multi-mode laser, comprising: a) a mass of lasant material having minimal birefringence between an input face and an opposite output face; b) an output coupler disposed towards said output face; c) at least one semi-conductor that, in response to the flow of electrical therethrough, pumps laser light into said input face of said lasant material for producing a population inversion in said lasant material; and d) input means for transmitting light from said semi-conductor to said lasant material and for reflecting laser light produced by said mass towards said output coupler to form an optical cavity for said lasant material, said optical cavity having optical length that is proportional to one-half the longitudinal mode spacing of said laser light; and e) birefringent means for optically retarding one of two polarization eigenstates relative to light in said cavity of the other polarization eigenstate by an amount to substantially eliminate phase coherence between said eigenstates, and for producing in said cavity two polarization eigenstates of substantially equal intensity and substantially single line multi-mode laser light. 12. The laser of claim 11, wherein said lasant material is Nd:YAG; and wherein said longitudinal mode spacing is on the order of 2 GHz. 13. The laser of claim 11, wherein said output coupler is coated to preferably reflect light at 1319 nm relative to light at 1338 nm. 14. The laser of claim 11, wherein said input means comprises a quarterwave plate having one face abutting said input face of said lasant material and having an opposite face abutting said input face. 15. The apparatus of claim 11, further including an aperture plate located between said output coupler and said input face of said lasant material. 16. In a communications system of the type using an external modulator which operates in response to an optical carrier supplied through an optical fiber, a method of providing a polarized optical carrier comprising the steps of: a) forming an optical cavity for a lasant material having an input face and an opposite output face by locating an output coupler disposed towards said output face and by locating a quarterwave plate having one face abutting said input face and having an opposite face that is coated for transmitting optical pumping light to said lasant material and for reflecting laser light produced by said lasant material towards said output coupler, b) providing a plurality of semi-conductors that, in response to the flow of electrical current therethrough, produces optical pumping light; c) pumping said lasant material using said semi-conductors to produce a population inversion in said lasant material; d) substantially reducing high order transverse modes in said cavity; and e) adjusting said optical cavity to have an optical length that is proportional to one-half the longitudinal mode spacing of said laser light and to produce standing wave substantially single frequency laser light characterized by two closely spaced apart orthogonal linear polarizations of about the same power level out of said output coupler for at least the some power levels of said semi-conductors. 17. The method of claim 16, wherein step (b) is performed by using an array of said semi-conductors distributed along a reference axis for end pumping said lasant material; and wherein step (a) is performed by using a quarterwave plate having an axis that is generally parallel to said reference axis of said array. 18. The method of claim 17, wherein step (d) is performed by locating in said optical cavity an aperture plate between said output coupler and said output face of said lasant material. 19. The method of claim 16, further including the steps of: f) optically connecting one end of a non-polarization maintaining optical fiber to said output coupler; and g) connecting a polarizer between said modulator and the other end of said optical fiber. 20. The method of claim 16, wherein step (a) is performed by using a lasant material formed from a crystal of Nd:YAG; wherein step (b) is performed by using semi-conductors that produce light at about 800 nm; and wherein step (c) is performed to produce laser light having a wavelength of substantially 1319 nm.

Inventors: ANTHON DOUGLAS W, EGGLESTON ALAN T, ANTHON, DOUGLAS, W., EGGLESTON, ALAN, T.
Applicants: AMOCO CORP, AMOCO CORPORATION
Application year: 1994
Main classification: H01S3
IPC codes: H01S3